1. Field of the Invention
The present invention relates generally to axial flow fans and, more particularly, to an axial flow fan which prevents deformation of blades even when rotated at high speed, thus promoting structural stability, and which achieves high efficiency and satisfactory capacity despite a low rotational frequency.
2. Background of the Related Art
As well known to those skilled in the art, axial flow fans are used to cool a heat exchanging medium circulating in, for example, a heat exchanger of a vehicle, such as a radiator or a condenser. As shown in FIG. 1, such an axial flow fan 10 includes a hub 20 which is coupled to an output shaft 52 of a drive unit 50 such as a motor, a plurality of blades 30 which are radially arranged along a circumferential outer surface of the hub 20, and a fan band 40 which couples outer ends of the blades 30 together to prevent deformation of the blades 30. The axial flow fan 10 having the above-mentioned construction is rotated by a rotational force transmitted from the drive unit 50 to the hub 20, so that air is blown in an axial direction by the rotation of the blades 30 of the axial flow fan 10.
Typically, the axial flow fan 10 is made of synthetic resin and formed as a single body. To efficiently guide air blown by the axial flow fan 10 to a heat exchanger, the axial flow fan 10 is assembled with a shroud 60 which is mounted to the heat exchanger. The shroud 60 to guide blown air includes a blast port having a predetermined size such that the axial flow fan 10 may be rotatably inserted into the shroud 60. The shroud 60 has a structure capable of supporting therein the motor 50 which is the drive unit.
As shown in FIG. 2, in each blade 30 of the conventional axial flow fan 10, both a leading edge (LE), which is an edge of the blade 30 in a rotational direction, and a trailing edge (TE), which is an edge of the blade 30 in a direction opposite the rotational direction, are curved in the direction opposite the rotational direction while extending from a blade root 32, which is a junction between the hub 20 and the blade 30, to an intermediate portion of the blade 30, thus forming a backward sweeping angle. Both the leading edge (LE) and the trailing edge (TE) of the blade 30 are integrated and curved in the rotational direction while extending from the intermediate portion of the blade 30 to the blade tip 34, which is the junction between the blade 30 and the fan band 40.
Such change of the sweeping angle of the blade 30 serves as an important factor to enhance the performance of the axial flow fan 10. However, it has been well-known that it is very difficult to achieve satisfactory air blowing efficiency and noise reduction.
In consideration of this, several axial flow fans were proposed in Korean Patent Laid-open Publication No. 2002-94183 and No. 2002-94184, which were filed by the inventor of the present invention.
As shown in FIGS. 3 and 4, an axial flow fan 10a of No. 2002-94183 includes a plurality of blades 30a each having a wave shape in which the sweeping angles of both a leading edge (LE) and a trailing edge (TE) alternate between forwards and backwards from a blade root 32a to a blade tip 34a. Furthermore, a chord length (CL), which is the length from the leading edge (LE) to the trailing edge (TE) of the blade 30a at the same radius, gradually increases from a blade root 32a to a blade tip 34a. In the drawings, the reference character “α” denotes the angle at which each blade 30a is disposed with respect to the horizon (H) when the axial flow fan 10a is level with the horizon (H). In the drawings, the reference numeral 20a denotes a hub, and 40a denotes a fan band.
As shown in FIGS. 5 and 6, an axial flow fan 10b of No. 2002-94184 includes a plurality of blades 30b each having a wave shape the same as that described for the axial flow fan 10a of No. 2002-94183. As well, the chord length (CL) of each blade 30b gradually increases from a blade root 32b to a blade tip 34b. Each blade 30b has a maximum backward sweeping angle at the blade root 32b and has a maximum forward sweeping angle at the blade tip 34b. In the drawings, the reference numeral 20b denotes a hub, and 40b denotes a fan band.
In the conventional axial flow fans 10a and 10b having a wave shape, air passing through the axial flow fan 10a, 10b is dispersed in a region between inflection points in which the direction of the sweeping angle changes. Therefore, concentration of the flowing air is prevented, thus improving air blowing efficiency and reducing noise.
However, in the conventional axial flow fans 10a and 10b, because the chord length (CL) gradually increases from the blade root 32a, 32b to the blade tip 34a, 34b, the blade tip 34a, 34b is structurally unstable. Accordingly, when the axial flow fan 10a, 10b is rotated at high speed, deformation of the blades 30a, 30b may occur. Particularly, the deformation of the blade tips 34a, 34b hampers the noise reducing function of the axial flow fan 10a, 10b. 
Furthermore, in the case of the axial flow fan 10b of No. 2002-94184, the angle (α1) between a line (L0), passing through both the center (O) of the hub 20b and an intersection point between the blade root 32b and a mid-chord line (ML), which connects middle points between the leading edge (LE) and the trailing edge (TE) of the blade 30b, and a line (L1), passing through both the center (O) of the hub 20b and an intersection point between the mid-chord line (ML) and the blade tip 34b, is smaller than an angle (α2) between the line (L0) and a line (L2), passing through both the center (O) of the hub 20b and a first inflection point (P1), defined at a first valley on the mid-chord line (ML), and is smaller than an angle (α3) between the line (L0) and a line (L3), passing through both the center (O) of the hub 20b and a second inflection point (P2) defined at a second valley on the mid-chord line (ML) (α1<α2, α3). In other words, the difference in width between each valley and opposite ends of the mid-chord line (ML) is large, and the forward sweeping angle of the blade tip 34b is excessively large. Thus, the conventional axial flow fan 10b must be increased in rotational frequency to achieve satisfactory capacity. As a result, there is difficulty in reducing noise occurring during the rotation of the axial flow fan 10b. 